Interpretive Summary: Lactoferrin (Lf), a bilobate iron-binding glycoprotein, is an antimicrobial component of milk and other exocrine secretions in mammals. The antimicrobial effects of lactoferrin are attributed to stimulation of immune cells, a strong capacity to bind iron, thereby reducing the availability of this essential element to bacteria, and a direct killing effect by binding to the surface of bacteria. Lactoferrin has been added to diets of fish as a prophylactic treatment against disease, but results have varied depending on the species of fish and test conditions. Our objective was to determine the effects of graded levels of bovine Lf (0, 200, 400, 800, or 1600 mg/kg diet) on growth performance, hematology, immune function, and the resistance of channel catfish to enteric septicemia and stress. Dietary Lf did not affect growth performance. However, level of Lf in diet had a significant effect on resistance of channel catfish to enteric septicemia (ESC). Fish fed the control diet (lacking Lf) were the most susceptible to ESC, but resistance to the disease improved in catfish fed diets supplemented with Lf and increased significantly as the level of Lf increased from 200 to 800 mg/kg in diet. ESC-related mortality did not appear to be related to measured immune responses but may have been related to plasma iron concentration, which decreased with increasing concentration of Lf in diet. Crowding stress produced significant increases on measured stress parameters from baseline values, but dietary Lf did not affect the stress response.

Technical Abstract:
Juvenile channel catfish (Ictalurus punctatus) were fed nutritionally complete, practical basal diets supplemented with bovine lactoferrin (Lf) at 0, 200, 400, 800, or 1600 mg/kg diet to apparent satiation twice daily for 5 weeks. Feed intake was significantly higher in fish fed diets supplemented with Lf compared to the control diet, but the increased feed intake did not translate to significant increases in growth performance (weight gain, feed efficiency ratio, or survival). Hemoglobin, white and red blood cell counts, and resistance to low-water stress were also not different among dietary groups (P > 0.05). However, levels of Lf in diets had a significant effect on survival of channel catfish following challenge with Edwardsiella ictaluri. Catfish fed 800 or 1600 mg/kg Lf had higher survival to E. ictaluri infection than the groups fed the control and 200 mg Lf diet. There was not a corresponding increase in activity of immune parameters (plasma lysozyme, hemolytic complement, and bactericidal activities or agglutination antibody titer) with addition of Lf to diets, but plasma iron decreased significantly in channel catfish fed increasing dietary concentrations of Lf. The ability of Lf to sequester iron, an essential nutrient required for the growth of bacteria, is regarded as one of its key antibacterial properties. The increased resistance to disease seemed to correspond with a decrease in plasma iron concentrations and not enhancement of non-specific or specific immune functions. The results from this work corroborate our previous research with Nile tilapia.